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In microcellular plastics, Nucleation is the first step in creating microcellular foams. In this process, a polymer is first saturated with a gas under high pressure, when the pressure is reduced, the gas becomes unstable in the polymer and nuclei (small bubbles) form. These nuclei then evolve into the cells that give the foam its structure. [1]
Nucleation happens continuously as the pressure drops, faster pressure reduction produces more bubbles because the gas becomes supersaturated more quickly. The cell size, cell density and cell morphology is established when gas diffusion limits growth. [1]
Factors that affect nucleation include: [1]
- Gas concentration
- more dissolved gas creates more bubbles
- Pressure drop rate
- faster depressurization encourages more nuclei
- Temperature and polymer properties
- softer polymers make it easier for bubbles to form
Studies were performed with ultrasound induced nucleation during microcellular foaming of Acrylonitrile butadiene styrene polymers. M.C.Guo studied nucleation under the shear action. As the shear enhanced, the cell size diminished and thereby increased the cell density in the foam. [2]